Chapter 38. Foot

Big Picture

The foot is connected to the leg by the ankle (talocrural) joint, which is an articulation between the tibia, fibula, and talus. The foot consists of 7 tarsal bones, 5 metatarsal bones, and 14 phalanges. Motion at the digits for abduction and adduction is defined by an imaginary line along the long axis of the second digit, unlike the hand in which the long axis runs along the third digit. Each digit, with the exception of the great toe, consists of three phalanges (proximal, middle, and distal); the great toe has two phalanges (proximal and distal). The articulations between the bones of the foot create multiple joints. The muscles that move these joints are divided into two groups, intrinsic and extrinsic foot muscles. The intrinsic muscles originate and attach in the foot, whereas the extrinsic muscles originate in the leg and insert in the foot, creating motion at multiple joints.

Joints of the Digits

The several bony articulations within the foot assist in accommodating uneven surfaces during weight-bearing activities. These motions of the foot are accomplished via the following joints (Figure 38-1):

• Metatarsophalangeal joints. Consist of articulations between the metatarsals and the proximal phalanges. The metatarsophalangeal joints allow flexion and extension and abduction and adduction.
• Interphalangeal joints. Consist of articulations between the phalanges, resulting in five proximal and four distal interphalangeal joints, which allow for flexion and extension.

Fascial Structures of the Foot (Figure 38-1)

• Plantar aponeurosis. Radiates from the calcaneus bone toward the digits. The plantar aponeurosis is a very thick fascia that invests the muscles of the plantar surface of the foot.
• Superior extensor retinaculum. Attaches from the anterior border of the fibula to the tibia, proximal to the ankle joint. The superior extensor retinaculum holds the tendons of the tibialis anterior, extensor hallucis longus, extensor digitorum longus, and fibularis (peroneus) tertius muscles next to the structures of the anterior ankle during contraction.
• Inferior extensor retinaculum. A “Y-shaped” structure that attaches laterally to the superior surface of the calcaneus bone and courses medially to attach to the medial malleolus and the medial side of the plantar aponeurosis. The inferior extensor retinaculum serves to tether the tendons of the fibularis (peroneus) tertius, extensor digitorum longus, extensor hallucis longus, and anterior tibialis muscles.
• Flexor retinaculum. Attaches between the medial malleolus and calcaneus bones, forming the roof of the tarsal tunnel. The tendons of the tibialis posterior, flexor digitorum longus, and flexor hallucis longus muscles as well as tibial nerve and posterior tibial artery pass through the tarsal tunnel to the enter into the plantar surface of the foot.
• Fibular retinacula. Tethers the tendons of the fibularis (peroneus) longus and brevis muscles on the lateral side of the ankle as they course inferior to the lateral malleolus bone.
• Dorsal digital expansions. An aponeurosis covering the dorsum of the digits that attaches proximally to the middle phalanx (digits 2–5) or proximal phalanx (digit 1), via the central band, and distally to the distal phalanx, via the lateral bands. The extensor digitorum longus and brevis muscles and the extensor hallucis longus and brevis muscles attach proximally and centrally to the dorsal digital expansion. The lumbricals and the dorsal and plantar interossei attach on the free edges. Because of the attachment of the muscles and the location of the dorsal digital expansion, the small intrinsic muscles produce flexion at the metatarsophalangeal joint while extending the interphalangeal joints.

Big Picture

Muscles that act on the joints of the foot can either be extrinsic (originating outside the foot) or intrinsic (originating within the foot), and they may act on a single joint or multiple joints. The result is movement of multiple joints used to accommodate uneven surfaces or for activities such as running or jumping. The intrinsic muscles are discussed in this section, and the extrinsic muscles are discussed in Chapter 37.

Foot Muscles

The medial plantar or lateral plantar nerves originate from the tibial nerve and innervate the plantar muscles of the foot. The deep fibular nerve innervates the muscles on the dorsal side. These foot muscles are divided into four layers on the plantar surface and one group on the dorsal surface (Table 38-1).

Layer 1 (Figure 38-2A)

• Abductor digiti minimi muscle. Attaches proximally to the medial and lateral tubercle of the calcaneal tuberosity; distally, it attaches to the lateral side of the base of proximal phalanx 5. The abductor digiti minimi muscle abducts and flexes digit 5. The lateral plantar nerve (tibial nerve) (S1, S2, S3) innervates this muscle.
• Flexor digitorum brevis muscle. Attaches proximally to the medial tubercle of the calcaneal tuberosity; distally, it attaches to both sides of the middle phalanges of digits 2 to 5. The flexor digitorum brevis muscle flexes the lateral four digits. The medial plantar nerve (tibial nerve) (S1 and S2) innerves this muscle.
• Abductor hallucis muscle. Attaches proximally to the medial tubercle of the calcaneal tuberosity and distally to the medial side of the base of the proximal phalanx 1. The abductor hallucis muscle abducts and flexes the great toe. The medial plantar nerve (tibial nerve) (S1 and S2) innervates this muscle.

Layer 2 (Figure 38-2B)

• Lumbricals. Attach proximally to the tendons of the flexor digitorum longus muscle and distally to the medial aspect of the dorsal digital expansions of digits 2 to 5. The lumbricals flex the proximal phalanges and extend the middle and distal phalanges. The medial plantar nerve (tibial nerve) innervates the first lumbrical, and the deep branch of the lateral plantar nerve (tibial nerve) innervates lumbricals 2 to 4 (S2 and S3).
• Quadratus plantae muscle. Attaches proximally to the medial and lateral margin of the plantar surface of the calcaneus bone and distally to the posterolateral margin of the tendon of the flexor digitorum longus muscle. The quadratus plantae muscle assists the flexor digitorum longus muscle in flexing the lateral four digits. The lateral plantar nerve (tibial nerve) (S1, S2, S3) innervates this muscle.

Layer 3 (Figure 38-2C)

• Flexor digiti minimi brevis muscle. Attaches proximally to the base of metatarsal 5 and distally to the base of proximal phalanx 5. The flexor digiti minimi brevis muscle flexes the proximal phalanx of digit 5. The superficial branch of the lateral plantar nerve (tibial nerve) (S2 and S3) innervates this muscle.
• Flexor hallucis brevis muscle. Attaches proximally to the plantar surface of the cuboid and lateral cuneiform bones and distally to both sides of the base of the proximal phalanx of the hallux (great toe, or digit 1). The flexor hallucis brevis muscle flexes the proximal phalanx of the hallux. The medial plantar nerve (tibial nerve) (S1 and S2) innervates this muscle.
• Adductor hallucis muscle. Proximally, the oblique head of this muscle attaches to the bases of metatarsals 2 to 4 and the transverse head attaches to metatarsophalangeal joints (plantar ligaments); distally, the entire muscle attaches to the lateral side of the base of phalanx of the hallux. The adductor hallucis muscle adducts the hallux. The deep branch of the lateral plantar nerve (tibial nerve) (S2 and S3) innervates this muscle.

Layer 4 (Figure 38-2D)

• Plantar interossei muscle. Attaches proximally to the bases and medial sides of metatarsals 3 to 5 and distally to the medial sides of the dorsal digital expansions of the proximal phalanges 3 to 5. The plantar interossei muscle adducts digits 3 to 5 and flexes the metatarsophalangeal joints of digits 3 to 5. The deep branch of the lateral plantar nerve (tibial nerve) (S2 and S3) innervates this muscle.
• Dorsal interossei muscle. Attaches proximally to the adjacent sides of metatarsals 1 to 5 and distally to the medial side of the dorsal digital expansions of proximal phalanx 2 and to the lateral sides of proximal phalanges 2 to 4. The dorsal interossei muscle abducts digits 2 to 4 and flexes the metatarso-phalangeal joints. The deep branch of the lateral plantar nerve (tibial nerve) (S2 and S3) innervates this muscle.

Big Picture

The tibial nerve enters the foot inferior to the medial malleolus through the tarsal tunnel, giving rise to the medial calcaneal branch (sensory). The nerve then bifurcates into the medial and lateral plantar nerves to supply motor and sensory innervation to the plantar surface of the foot. The deep fibular nerve supplies motor innervation to the dorsum of the foot as well as sensory innervation to a small area between the first and second digits. Sensory innervation to the medial and lateral sides of the foot is provided by the saphenous nerve (from the femoral nerve) and the sural nerve (from the tibial nerve) (Figure 38-3A and B).

Medial Calcaneal Nerve

The medial calcaneal nerve originates from the tibial nerve as it courses through the tarsal tunnel, providing sensory innervation to the posterior plantar surface of the foot over the heel. The medial calcaneal branch does not provide any motor innervation.

Medial Plantar Nerve

The medial plantar nerve is one of two terminal branches of the tibial nerve and travels deep to the abductor hallucis muscle (the other terminal branch is the lateral plantar nerve). The medial plantar nerve courses adjacent to the flexor digitorum brevis muscle, providing motor innervation to the first lumbrical and the abductor hallucis, flexor hallucis brevis, and flexor digitorum brevis muscles. The medial plantar nerve also provides sensory innervation to the medial plantar surface of the foot.

Lateral Plantar Nerve

The lateral plantar nerve is one of the terminal branches of the tibial nerve that courses from the medial side of the foot laterally toward the head of metatarsal 5, deep to the flexor digitorum brevis muscle. The lateral plantar nerve continues forward and bifurcates into a deep branch and a superficial branch. This nerve provides motor innervation to lumbricals 2 to 4 and the plantar and dorsal interossei, adductor hallucis, quadratus plantae, flexor digiti minimi brevis, and abductor digiti minimi muscles. The lateral plantar nerve provides sensory innervation to the lateral aspect of the plantar surface of the foot.

Big Picture

The posterior tibial artery courses through the tarsal tunnel, inferior to the medial malleolus bone, and bifurcates into the medial and lateral plantar arteries to supply blood to the plantar surface of the foot. On the dorsal side, the anterior tibial artery crosses the anterior ankle joint as the dorsalis pedis artery, which supplies blood to the dorsal side of the foot. The blood of the foot is returned to the femoral veins via the deep and superficial venous systems. The deep venous system follows the arteries.

Dorsalis Pedis Artery

The dorsalis pedis artery courses distally on the dorsum of the foot and provides the following branches:

• Medial and lateral tarsal arteries. Branch medially and laterally from the dorsalis pedis artery, supplying blood to the tarsals and adjacent structures, and contributes to the arterial network surrounding the ankle.
• Arcuate artery. A lateral branch from the dorsalis pedis artery that branches into the dorsal metatarsal arteries, providing blood to adjacent structures of metatarsals and digits 2 to 5. In addition, the arcuate artery communicates with the plantar surface of the foot through perforating branches.
• First dorsal metatarsal. The dorsalis pedis artery courses distally to the base of metatarsals 1 and 2 and bifurcates into the first dorsal metatarsal artery and the deep plantar artery, which perforates through to the plantar surface of the foot. The first dorsal metatarsal artery supplies blood to adjacent structures around the first metatarsal and the great toe.

Lateral Plantar Artery

The lateral plantar artery originates from the posterior tibial artery inferior to the medial malleolus bone and courses laterally between the quadratus plantae and flexor digitorum brevis muscles. The lateral plantar artery courses distally along the medial edge of the abductor digiti minimi muscle and curves medially, forming the deep plantar arch. The deep plantar arch provides blood to the adjacent structures of metatarsals and digits 2 to 5, the lateral side of metatarsal 1, and the great toes, via plantar metatarsal arteries and digital branches. The terminal end of the deep plantar arch joins the deep plantar artery of the dorsalis pedis artery.

Medial Plantar Artery

The medial plantar artery originates from the posterior tibial artery, inferior to the medial malleolus bone. It courses distally along the medial edge of the abductor hallucis muscle, supplying blood to the adjacent structures on the medial side of the first metatarsal and the great toe, via the first plantar metatarsal artery and digital branches.

Veins of the Foot

The foot contains deep and superficial venous systems. The deep veins are named according to the arteries they follow. The superficial venous system drains into the dorsal venous arch. The medial side of the foot drains into the great saphenous vein from the dorsal venous arch and the lateral side of the foot into the small saphenous vein.